1. Field of the Invention
The present invention relates to an image forming apparatus, and particularly, to an image forming apparatus having a configuration that corrects skew-feeding of a sheet and a position of a sheet in the width direction.
2. Description of the Related Art
Hitherto, image forming apparatuses such as copying machines, printers, and facsimiles are equipped with a sheet conveying apparatus that conveys a sheet to an image forming portion. Here, when skew-feeding or displacement in a position (transverse registration position) in the width direction perpendicular to the sheet conveying direction occurs in the sheet in conveying the sheet toward the image forming portion, an image is formed on the sheet while the image position is displaced. Therefore, in the sheet conveying apparatus of the related art, a skew-feeding correcting portion which corrects the skew-feeding of the sheet and aligns the transverse registration position is provided at the upstream side of the image forming portion in the sheet conveying direction. As an example of the skew-feeding correcting portion, there is proposed a skew-feeding correcting portion which corrects a displacement in a position of a side end of a sheet during a conveying operation based on a side registration (see U.S. Pat. No. 6,273,418).
In the skew-feeding correcting portion which aligns the sheet based on the side registration, a reference member with which one side end of the sheet collides is provided along the sheet conveying direction at one side of the sheet conveying path, and a plurality of skew-conveying rollers is arranged on the sheet conveying path. Then, the sheet which is conveyed by the sheet conveying roller provided at the upstream side in the sheet conveying direction is skew-conveyed by the skew-conveying roller toward the reference member, and one side end of the sheet collides with the reference member, so that the displacement in the width direction and the inclination of the side end of the sheet are both corrected.
Here, in the skew-feeding correcting portion which uses the skew-conveying roller, if the sheet pressing force against the reference surface (the transverse approaching force by the skew-conveying roller) is too strong when the side end of the sheet collides with the reference surface of the reference member, the sheet is warped, thereby leading to a paper jam or degradation in correction precision in the sheet. For this reason, in the skew-feeding correcting portions of the related art, there is also proposed a skew-feeding correcting portion which moves a side end of a sheet along a reference surface without causing any warpage in the sheet by adjusting a force of pressing the side end of the sheet against the reference surface with a change in the nipping pressure of a skew-conveying roller (see U.S. Pat. No. 5,253,862).
Here, in the image forming apparatus with the skew-feeding correcting portion of the related art, the force of pressing the side end of the sheet against the reference surface can be adjusted, but when the stiffness of the sheet is low, the skew-feeding correction may not be performed with high precision just by the adjustment of the pressing force. For example, in a case where the sheet is a super-thin sheet (a coated sheet having a basis weight which is less than 70 gsm), the stiffness of the sheet is low. Further, in a hot and humid environment, the stiffness of the sheet is degraded.
In general, since the thin coated sheet is easily wound around the fixing roller in the fixing portion to thereby cause a jam, the winding around the fixing roller is prevented by passing the sheet S so that the fiber-orientation direction is parallel to the sheet conveying direction as illustrated in FIG. 18A. Furthermore, the fiber-orientation direction indicates a direction of the fiber of the sheet, and the stiffness of the sheet or the easy occurrence of the curl is different in the fiber-orientation direction. When the sheet is conveyed in the fiber-orientation direction in this way, in a case of a duplex mode, the sheet passing through the fixing portion and having an image formed on both faces follows the fiber-orientation, that is, the sheet is curled in parallel to the sheet conveying direction. Then, when the curl (hereinafter, referred to as a gutter-shaped curl) parallel to the sheet conveying direction occurs in the sheet S, the stiffness of the sheet S in the width direction is degraded.
For this reason, as illustrated in FIG. 18B, when the sheet S having an image formed on both faces collides with a reference member 31 by pairs of skew-conveying rollers 32a to 32c, the sheet S may be warped in the collision direction. In this case, the skew-feeding correction precision is degraded compared to the case of forming an image on one face of the sheet. That is, in a case of a super-thin coated sheet, the side end position changes by the warpage amount of the sheet between a case of forming an image on a first face and a case of forming an image on a second face. For this reason, even when the sheet collides with the same reference surface, the positions of the front and rear faces in the width direction may not match each other. As a result, when an image is formed on both faces of the sheet, the image forming positions of both faces of the sheet are displaced in the width direction, so that the image quality is degraded.
Therefore, the invention is made in view of these circumstances, and it is desirable to provide an image forming apparatus capable of suppressing a displacement between image forming positions of both faces of a sheet in the width direction even in a sheet having a low stiffness.